– In this video, we’ll
talk just a little bit about the anatomy of
the large intestine and the things that go on
in the large intestine. Looking at it grossly,
the large intestine forms sort of a picture frame
around the small intestine, circling the entire
abdominal cavity. You have the first part of
the small intestine, which is the cecum, which hangs,
actually, a little bit below the ileocecal valve. Which is where the partially–
well, the pretty much completely digested
at this point, mass from the small intestine
where that stuff is coming into the large intestine. Coming off the end of the cecum,
you have the vermiform appendix, which is a little pocket
of lymphoid tissue. It’s a hollow tube that looks
a little bit like a worm. That’s why it’s
called the vermiform. That’s basically what
vermiform means, is worm-like. And there’s just a concentration
of lymphoid follicles in here. This part often can trap
bacteria, can get blocked, and become deeply inflamed. At which point, it
needs to be removed. And acute appendicitis, acute
inflammation of the appendix, can actually be
very, very dangerous. Because if the
appendix ruptures, you’re just, basically,
spewing infection all over the abdominal cavity. So the trick is to remove
before it ruptures. Moving up from the cecum
you have the ascending colon. And it’s called
the ascending colon because if you’re tracing
the path that that food waste mass moves, it moves up
the abdomen at this point. Then you have the
transverse colon. Transverse because food is
moving across the abdomen. Descending colon. The sigmoid colon,
because it’s kind of roughly S-shaped and
sigma is the Greek S. And then emptying down
into the rectum, kind of a storage unit
for waste before it’s pushed out the anal canal. Most of what’s absorbed in
the large intestine is water. But I do want to make a
note that about 90% to 95% of the water in your
chyme is reabsorbed in the small intestine. And most of the
remainder is absorbed from the large intestine. So under normal
healthy conditions, by the time feces
leaves the body, there’s only about
1% to 2% of the water that was originally
in that chyme that is still in the feces. Of course, if you are
having diarrhea issues and food is moving through– that food waste is moving
through your digestive tract too quickly to
reabsorb much water, then you’re going to have a
higher proportion of water in your feces. The large intestine itself
is about 5 feet long. It’s called the large intestine
because it has a wider diameter than the small intestine. It’s about 10 centimeters wide,
versus the small intestine is about 2 to 3
centimeters wide. And there is– the longitudinal
layer of smooth muscle is reduced to three
bands of muscle. And these are called
the taenia coli. So there’s not really
two complete layers of muscularis externa
around the large intestine. You just have the taenia coli
as that longitudinal layer. And these bands of smooth muscle
are, under normal conditions, under resting conditions,
they’re a little bit contracted. And they pull the intestine
into the sac-like pockets called haustra. You also have little fat-filled
pockets of visceral peritoneum hanging off the taenia coli. These are the
epiploic appendages. Not really entirely
certain what the function of the epiploic appendages
are, if there is a function, but they’re there. And they are recognizable
on the large intestine. You have strong peristaltic
waves, called mass movements, that occur a few times
a day pushing feces through the large intestine. You also have
segmentation happening in the large intestine. And the segmentation is
called haustral churning because it mixes the contents
of neighboring haustra. Haustra is the plural form,
haustrum is the singular. There’s a reflex that empties
chyme into the large intestine, and that’s called the
gastroileal reflex. Gastrin, when it’s released
by the stomach, when the stomach is
distended– when you’ve got food emptying into the
stomach, the stomach stretches. That triggers the
release of gastrin. And one of the
effects of gastrin– I mean, it acts on
the stomach as well. But gastrin also triggers
opening of the ileocecal valve. So when you have new
bunches of food moving into the stomach, that actually
triggers movement of food out of the small intestine
into the large intestine as this ileocecal valve, the
sphincter here, opens up. You also have sphincters
acting down here at the anus. There are two
sphincters at the anus. An internal
involuntary sphincter and an external
voluntary sphincter. So with peristalsis, those
mass movements force feces out of the sigmoid
colon into the rectum. And as the rectum
distends, as it stretches filling with feces, that
triggers a reflexive opening of the internal sphincter,
but actually closes the external sphincter. So to actually
expel that waste, it requires a voluntary relaxing
of the external sphincter. And that’s usually a fair– a somewhat conscious response. But if the pressure
gets too high– if the pressure gets
too high in the rectum, the external sphincter
will relax involuntarily. This is what happen– this
is how infants defecate. Before infants learn
that voluntary relaxation of the external
sphincter, they just rely on that
accumulation of pressure in the rectum to force the
external sphincter to relax. This is a cadaver view
of the large intestine. You can see the cecum down here,
with the vermiform appendix coming off of it. The ascending colon, moving up. Transverse colon. And then the descending
colon and the sigmoid colon down there leading into
the rectum and anal canal. These sharp right angle
turns that the colon makes are called flexures. So if you hear reference
to colonic flexures, that’s what they’re talking
about, these sharp turns. Now since water is being
reabsorbed from the feces as they move through
the large intestine, those feces are getting
drier and drier as they move. Which means that lubricating
the passage of those feces through the large intestine
is really important. And one of the identifying
characteristics of the large intestine
under the microscope is enormous numbers
of goblet cells. Remember, goblet cells are
these mucus-secreting individual columnar cells that,
they swell with mucus at the apical surface. And that’s why they’re
given the name goblet cell. So you have goblet cells all
along the large intestine just lining the epithelium
of the large intestine. And that’s one way to
identify the large intestine histologically. And these goblet
cells secrete mucus that ease the passage of feces
through the large intestine. Also in the large
intestine, you have the majority of the
gastrointestinal microflora. So you have an enormous
number of bacteria that make your digestive
tract their home. And they are beneficial. They’re actually helpful. So you want them there. You want to keep them happy. You want to feed them lots of
fiber that they can feed on. And the bacteria come
entering through the anus. There are a few that
manage to make it through the hostile
environment of the stomach, and the acidity of the stomach,
and the digestive enzymes in the small intestine. But they usually set up
shop in the large intestine and they help break down
indigestible carbohydrates. So, basically, the fiber in your
food that your own digestive enzymes can’t break
down, some of that can be broken down by bacteria
in your large intestine. So there is a little
bit of digestion that takes place in
the large intestine. But it’s not your
enzymes doing it, it’s your bacteria. Now, unfortunately, when
bacteria are breaking down this indigestible
carbohydrate material, it tends to release gases. Which, depending on what kinds
of bacteria you have may, can be pretty smelly. That’s why high fiber
foods, like beans, tend to leave people
feeling kind of gassy. Because their bacteria
are going to town on the fiber in those
beans and producing gas. These bacteria also synthesize
a few vitamins for us. Some B-complex vitamins and
vitamin K. So biotin, folate, pantothenic acid, riboflavin,
B6, B12, thiamine, these are all synthesized by
your bacterial microflora. And these vitamins that are made
by the bacterial microflora, as well as the
short-chain fatty acids and other products
of fiber breakdown, these are absorbed in
the large intestine. So there’s a small amount
of absorption happening in the large intestine. There’s water absorption,
and there’s also absorption of these nutrients
that are made by your bacteria. Newborns don’t ha– their gut
bacteria isn’t established yet. It takes a bit of time
for the gut bacteria to really become well– for their gut to
become well-colonized. And this is why,
often, newborns will be given an oral dose of
vitamin K. Vitamin K is really essential for blood clotting. And so to prevent bleeding
disorders in newborns, they’ll get a dose of
vitamin K so that their body can make clotting factors. And that will hold them
until their bacteria, their gut bacteria become
established and start making vitamin K for them. So a few things about
the large intestine you should be able to discuss
after watching this video and studying it. You should be able to talk
about the different regions. So the cecum, ascending colon,
transverse colon, descending colon, sigmoid colon. You should be able to talk about
the digestive activities that are carried out in
the large intestine, including the activities
that the bacteria do. And you should be able to talk
a little bit about the vermiform appendix, both what it normally
does as lymphoid tissue and what can go wrong with it.

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